Microsoft made some interesting design decisions with UWP and this causes build issues with Gaia.
Unfortunately these issues are all related to file handling, and as Gaia uses files extensively it is not a quick fix.
The work around is to export the Gaia scripts directory as a package, then delete them and make your build. Then re-import the scripts when you are ready to work in Gaia again.
The re-import process is very fast as the exported directory is tiny.
I will redo all of this for Gaia 2.
WORKAROUND UPDATE (as fed back to me by a customer - many thanks Dr Hayashida!):
Change the project target back to PC, Mac & Linux Standalone build and run.
The change back to UWP and visualise on your headset.